Electronic circuits which are connected to a large network, such as an electrical power or a telecommunications network need protection from overvoltage conditions to which they may be subjected as a result of lightning strokes, switching surges or other events which typically result in overvoltage surge propagation in such networks. What may be adequate protection for one type of electronic circuit may be entirely inadequate for another one with elements more susceptible to this sort of damage.
Circuits which are realized in CMOS technology are particularly susceptible to damage by overvoltages and are at the same time difficult to protect from them. Bipolar device circuits can be protected by a a zener diode in conjunction with a silicon controlled rectifier (SCR) latch. The latch is formed by an NPN and a PNP transistor and appropriate resistors which are connected between their collectors and a positive voltage supply node. When such transistors are formed in a CMOS integrated circuit (IC), however, their collectors are necessarily tied directly to the positive voltage supply node and are therefore not accessible for the connection of resistors. On the other hand, the MOS devices, which could be readily formed in the circuit, are not suited for carrying the currents involved in that function